1. Field of the Invention
The present invention relates to scanable offset antenna arrangements which produce at the exit aperture thereof a large image of a small feed array with minimal aberrations and, more particularly, to scanable offset antenna arrangements wherein a main parabolic reflector and a subreflector are disposed coaxially to achieve both paraxial and geometric surface confocality while positioning the feed array at the conjugate plane relative to the exit aperture of the reflector system.
2. Description of the Prior Art
Limited scanning offset feed antenna arrangements have been devised for, inter alia, radar systems and now also suggested for satellite communication systems both for the antennas of the ground stations and those of the satellite. One such arrangement is disclosed in U.S. Pat. No. 3,500,427, issued to S. Landesman et al on Mar. 10, 1970 where the arrangement comprises a panel of radiating elements capable of scanning through an angle and an offset reflector system comprising a parabolic main reflector and an elliptical subreflector for producing a high gain. The reflectors are described as substantially confocal with the panel of radiating elements being disposed at one foci of the elliptical subreflector to compensate for abberations in the optical system. The reflectors, however, are not geometrically confocal in that the focus of the paraboloid main reflector does not coincide with either of the foci of the elliptical subreflector. As a consequence, a plane wave incident on the paraboloid main reflector in the direction of its axis is not transformed into a plane wave after two reflections and the feed array illumination can be considered to be a plane wave only in the vicinity of the array center.
Another scanable offset antenna arrangement is disclosed in the publication entitled "Limited Electronic Scanning with an Offset-Feed Near-Field Gregorian System" by W. D. Fitzgerald in Technical Report 486 of the Lincoln Laboratory (MIT), Sept. 24, 1971. There, a near-field Gregorian antenna is disclosed which uses offset confocal and coaxial sectional paraboloid reflectors and a relatively small planar-array feed.
Although the various prior art arrangements have rays which converge to a common focus between the two reflectors for producing wavefronts after each reflection which gives very limited imaging of the feed array, such arrangements generally introduce spherical aberrations and provide planar wavefronts over only very small portions of the aperture or scan angle. Additionally, in such arrangements phase aberrations are improved by generally shaping the subreflector to reduce imperfections in the main reflector. The problem remaining in the prior art, therefore, is to provide an antenna arrangement which provides both substantially improved phase characteristics and imaging without requiring specially designed subreflectors or highly accurate surface geometries in the main reflector and subreflector to be used.